1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a hard disk drive with a shock event logging capability.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk and data is written to or read from each track by positioning the transducer over the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical hard disk drive further comprises a head stack assembly (HSA) that includes a transducer, a pivotally mounted actuator arm for supporting the transducer, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil which results in the permanent magnet applying a force onto the coil which is then transferred to the actuator arm in the form of a torque. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to reposition the transducer by first directing the control current through the coil so as to angularly accelerate the actuator arm in a first direction and then reversing the control current so as to angularly decelerate the actuator arm.
The rotation of the disk and positioning of the transducer relative to the disk are performed in manners that rely on tight tolerances between parts that move relative to each other at high speeds. Faster rotation of the disk generally facilitates faster data retrieval and writing. Smaller distance between the transducer and the disk surface, and improvements in the transducer performance generally facilitate increase in areal density of data storable on the disk surface. One consequence of increase in areal data density is a closer spacing between adjacent tracks. Thus, as technology permits, the hard disk drives operate at faster speeds in general, and tolerances between moving parts, and between functional features such as tracks, become smaller.
One of the consequences of operating disk drives at small tolerances is that moving parts or features in close proximity are susceptible to unwanted physical contact or intrusion when the disk drive is subjected to a disruptive movement such as a shock. While disk drives are engineered to withstand a certain level of shock, many disk drives are subjected to potentially damaging shocks during operation. This is particularly true with disk drives associated with portable computers, such as laptop computers, that are moved about considerably more than a stationary computer.
When the disk drive receives a shock, the resulting ‘damage’ may range from a simple missed data retrieval that can be remedied easily during subsequent rotations of the disk, to a physical damage to the disk surface that results in permanent loss of data recorded thereon. Another common damage that can occur due to a shock is that the transducer can be bumped from its assigned track and intrude into neighboring tracks, while a write signal is being applied to the transducer. Such mishap results in unwanted writing of data on the wrong track, thus causing a loss of data on that written-over portion.
A shock may also occur when the transducer is in the process of reading data from the disk. A sudden displacement of the transducer to another track may yield inclusion of wrong data segment in the data output from the disk drive. While such a situation does not damage the data itself on the disk, inadvertent passing of wrong data to the computer may have serious negative effects in many aspects.
To prevent such writing and reading of data on the wrong track, the servo controller receives position error signals (PES) from the relative positioning of the transducer to the servo wedges. If the PES indicates that the transducer is off track, the transducer is brought back in line with the track in a manner well known in the art. As is also known in the art, when the transducer is off track by more than a predetermined amount, a write-unsafe (WUS) condition exists, wherein application of write signal in such a condition can damage the data recorded on the adjacent track. The WUS condition is typically triggered when the transducer deviates from its assigned track by more than 16% of the track width.
In addition to triggering the WUS condition by excessive PES, some disk drives include a shock sensor that senses shocks independently. When the shock sensor is subjected to a shock with a magnitude larger than a predetermined threshold, the shock sensor generates a signal functionally similar to the WUS signal.
Other methods of detecting shocks to the hard disk drive include a back-emf signal processor or similar devices that detect unexpected movements of the transducer. As is known in the art, motion of a coil, such as that of the VCM, with respect to a changing magnetic flux generates a voltage across the leads of the coil so as to oppose the flux change. This effect, known as back-emf, exists in the VCM coil whether a control current is being applied to the VCM or not. Thus, the back-emf signal can be detected so as to determine if the transducer's movement is an unexpected movement due to effects such as a shock.
Shock detection systems, such as those referred to above, typically prevent erroneous writing or reading of data. As such, the function of shock detection systems generally does not provide additional function beyond the prevention of erroneous operations. For example, a disk drive may be damaged as a result of a shock. While the shock detection system might have prevented writing or reading of data during the shock event, other components of the disk drive may have been damaged. When the disk drive is being serviced due to such damage, a service provider may not have knowledge of what might have caused the damage.
From the foregoing, it will be appreciated that there is a need for a shock event logging system in hard disk drives. In particular, there is a need for a shock event logging system that permits improved diagnosis of shock related damages to the disk drive.